1. Field of the Invention
The present invention relates to a drop on demand or continuous ink jet print head having an orifice plate the dimensions of which may be electronically controlled during the printing operation by piezoelectric displacement, thereby controlling the size of the ejected ink droplets.
2. Description of the Related Art
Ink jet printers are one of the most popular printer devices for personal or industrial use. Most ink jet printing systems may be classified as either a "continuous jet" type ink jet printing system or a "drop on demand" type ink jet printing system. In a continuous jet system, a continuous stream of ink is ejected from the print head, and broken into a series of discrete ink droplets, usually by a piezoelectric transducer. The ink droplets are directed either toward or away from the substrate on which the printed image is desired, according to the image data communicated to the printing system. The ink droplets that are directed away from the substrate are reclaimed and reintroduced into the continuous stream of ink.
In a drop on demand system, the print head contains one or more ink chambers filled with ink at or near atmospheric pressure. At this pressure, ink does not eject from the ink chambers even though each ink chamber contains an orifice sufficiently sized for the ejection of the ink. Each ink chamber is coupled to an electromechanical or electrothermal driver actuator that is used to create transient and localized increases in the pressure, according to the print image data. The increase of pressure accelerates the ink in the chamber which momentarily overcomes the threshold resistance needed to eject a droplet of ink through the orifice, thereby ejecting an ink droplet through the orifice. The pressure increases are generally created by a piezoelectric driver actuator or a "bubble jet" driver actuator. In the "bubble jet" system, an air bubble is formed adjacent to a heating element located at the surface of the ink chamber wall. The air bubble grows until it overcomes the surface tension of the chamber wall and displaces the surrounding ink until it overcomes the threshold resistance described above and is ejected through the orifice.
Generally speaking, the resolution of an ink jet printer is limited by the size of the ink droplets produced by the print head and the discrete positioning capability of the print head. The resolution may be increased by utilizing smaller orifices, but smaller orifice diameters result in slower print speeds. Since a printer for home, office or industrial use is normally asked to print high resolution at times and low resolution at other times, a single orifice diameter is insufficient. Several approaches and inventions therefore have been directed at producing a print head with arrays of orifices of more than one diameter.
A common approach to this problem involves software techniques known as half toning. Half toning methods direct the printer to overlay two or more ink droplets, either at a single print head position or at minute displacements from a single print head position. These approaches create different shades of ink impressions on the substrate, but utilize the single, fixed orifice diameter available in the attached print head. Therefore, half toning methods typically are slow and produce only marginal improvements over standard non-impact printing methods. Fixed orifice devices have also been designed to produce ink droplets of variable sizes by varying the driver actuator signals. See, U.S. Pat. No. 5,124,716 (to Roy, et al.), U.S. Pat. No. 4,513,299 (to Lee, et al.), and U.S. Pat. No. 5,495,270 (to Burr, et al.). To date, however, the variation in ink droplet sizes in such single-orifice-diameter systems is insufficient for the various tasks required of home, office, and industrial printers.
U.S. Pat. No. 5,208,605 to Drake discloses a dual array of orifices wherein one array has a higher resolution than the other to provide the capability of producing letter quality print. Drake therefore utilizes orifices of more than one diameter to provide both high-resolution print capabilities and speed when high-resolution is not required. One basic problem with Drake, however, is that the print head may become quite bulky and still can produce only two ink droplet sizes. Also, the configuration of Drake's print head appears to require additional translational movement of the print head when utilizing both orifice arrays in combination, thereby reducing the print speed and the accuracy of the positioning of the ink droplets.
An orifice plate containing four orifices of four different diameters is disclosed in U.S. Pat. No. 5,077,565 to Shibaike et al. For a given application, Shibaike positions the orifice of the desired diameter over the ink chamber by sliding the entire orifice plate via an electrostatic force. Shibaike discloses a complicated and costly manufacture process creating and array of "micro-machine" slider plates through lithography and etching on a semiconductor substrate.
U.S. Pat. No. 5,430,470 to Stortz discloses an ink jet system that replaces the electromechanical or electrothermal driver actuators with a constant high-pressure ink chamber system. Instead of employing driver actuators to drive the ink through the orifices, Stortz employs an array of piezoelectric shutters, opening same momentarily to allow an ink droplet to pass through, according to the print image data. The problems with the Stortz configuration are many, most of which stem from Stortz's attempt to design a high-pressure print head system. Instead of operating for the most part at atmospheric pressure, Stortz's disclosure operate under constant pressure. This requires a leak-proof seal at all times, except when the control signal voltage commands the piezoelectric shutter to open to allow the passage of ink, according to the print image data. Thus, a high-pressure system results in costly manufacture, power inefficiencies, and potential safety and leakage problems. Moreover, the Stortz system results in a limitation in controlling the exiting ink droplets producing unwanted tail features and satellite droplets that trail the intended ink droplet due to the shear forces present at the open shutter surfaces as the ink passes through under pressure.